1. Field of the Invention
The present invention relates to a focus detection device for a camera comprising a light projection means for projecting light beams onto an object through an objective lens mounted on the camera, a light receiving means for receiving light beams reflected by the object through the objective lens and a focus detection means for detecting the condition of focusing from outputs of said light receiving means.
2. Description of the Prior Art
Generally speaking, one of the important factors required of a focus detection device for a single lens reflex camera is an applicability for a variety of interchangeable objective lens each having an individual F-number and an individual focal length. It is also an important factor that the focus detection can be carried out irrespective of the brightness of an ambient light and/or an object.
There is proposed a focus detection system in which an auxiliary illuminating means is installed in an electronic flash device to be mounted on the camera for projecting light beams to an object when the circumference is too dark into detecting the focusing (See U.S. Ser. No. 807,642).
FIG. 1(a) shows a conventional focus detection system schematically.
As shown in FIG. 1(a), a focus detection module 3 is provided in a camera body 1 with an objective lens 2 for receiving an image of an object through the objective lens to detect the focusing condition of the objective lens automatically. As indicated by two lines Su and Sl in FIG. 1(a), the area in which the focusing can be detected by the focus detection module is limited to an area defined between two lines Su and Sl being symmetric to each other with respect to the optical axis of the objective lens.
In an electronic flash device 4 being mounted on the camera body 1, there is provided an auxiliary illuminating means 5. The auxiliary illuminating means 5 projects illuminating light beams onto an object to which the focusing condition is detected, and the focusing condition is detected by receiving light beams reflected from the object when the circumference is dark. The auxiliary illuminating means 5 is arranged to have a distance of Bo to the optical axis OA of the objective lens and the optical axis thereof is so inclined as to make an angle of .theta. with the optical axis OA of the latter. Further, the width of light beams emitted therefrom is so restricted as to have a width as is defined between two dotted lines Ru and Rl.
In such a focus detection system as mentioned above, a range being illuminated by the light of said auxiliary illuminating means is limited to a range indicated by l in FIG. 1(b) defined between Lmin and Lmax, since the optical axis OB of the auxiliary illuminating means is inclined to the optical axis OA of the objective lens. Although it is not impossible to elongate the range l by widening the projection angle of the illuminating light beams, this invites insufficiency in the amount of the illuminating light. Further, this system has a disadvantage in that it is not available if one does not bring the electronic flash device.
In JP-A No. 155832/1979, there is proposed another focus detection system in which an illuminating means is installed within the camera body which projects illuminating light beams onto an object through a predetermined area of the objective lens. The light reflected from the object is received, through another restricted area of the objective lens, by a light receiving device installed in the camera body to detect the focusing condition. Although this system can elliminate such disadvantages mentioned above, it invites other problems such that the projected light beams are reflected by surfaces of lenses comprising the objective lens to cause ghost images and/or flares which negatively influence the focus detection.
Contrary to the above, in JP-A No. 22210/1982, there is proposed an optical system for focus detection in which an optical axis for projecting light beams through a predetermined area of the objective lens and an optical axis for receiving the reflected light through another area thereof are set so as not to be symmetric to the principle plane of the objective lens with respect to a crossing point on the optical axis of the objective lens with said principle plane. According to this optical system, such problems as mentioned above are eliminated. Also, due to said optical arrangement in regard to the optical axes for projection and for receiving, ghost images and flares are removed, as far as the focus detection is concerned.
Since there is such a relation that ghosts and flares due to reflections between or among surfaces of the lens are generated symmetrically with respect to the optical axis of the objective lens, it is better to avoid said relations in order to remove bad influences due to ghosts and flares. One of the important points to be considered when said optical system is to be applied to a single lens reflex camera with a variety of interchangeable lens is that the range in which ghosts and flares are generated symmetrically with respect to a point on the optical axis of varied according to the construction of the objective lens, the driving amount of the objective lens for focusing and so on. This depends on differences among optical constructions of objective lenses. Since there is a restriction with respect to the focus detectable distance, as mentioned above, in a so-called active focus detecting system (hereinafter referred to as the active AF) in which light beams are projected onto an object from the camera and the reflected light is received to detect the focusing condition, said system is not available to an interchangeable objective lens having a relatively long focal length. Therefore, it is better to use a so-called passive auto-focus detecting system (hereinafter referred to as the passive AF) which detects the focusing condition with use of ambient light and which has no restriction with respect to the focus detectable distance together with the active AF.
In any system of the active or passive AF, it is necessary that the system can be adapted to any of a variety of interchangeable objective lenses having different F-numbers and focal lengths respectively. In order to achieve this result, the optical axis for receiving the reflected light should be set close to the optical axis of the objective lens. However, such an arrangement is apt to be affected by ghosts and flares upon using the active AF.